Graphite is an allotrope of carbon in which the atoms are arranged in large sheets of fused six member rings. Single sheets of carbon that make up graphite are known as graphene or graphene sheets. Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a building block for graphitic materials of all other dimensionalities. Graphene or graphene sheets are the single sheets of carbon that make up graphite, i.e., is graphite in a layered form.
Graphite has many useful properties including a low coefficient of friction, good electrical conductivity, and high thermal resistance [1]. Graphite does not, however, interact well with water, as it is insoluble and difficult to suspend. Graphite oxide, on the other hand, contains oxygen attached to the layers as epoxy bridges and hydroxyl groups [2-4]. The properties of graphite oxidediffer significantly from graphite, for example, graphite oxide is an electrical insulator and thermally decomposes. Graphite oxide is also significantly more hydrophilic than graphite, providing complete exfoliation upon suspension in water [2, 4-6].
Graphite oxide (C2O) was synthesized as early as 1860 by Benjamin C. Brodie by treating graphite with a mixture of potassium chloride and fuming nitric acid [14]. Hummers and Offeman later developed a quicker, safer method of preparing graphite oxide than that which was used by Brodie, later termed “Hummer's method”. Graphite oxide is most commonly produced using Hummers method, which includes using a mixture of sulfuric acid (H2SO4), sulfuric nitrate (NaNO3), and potassium permanganate (KMnO4) [15]. Unfortunately, these methods of graphite oxide synthesis require large amounts of concentrated acid, powerful oxidizers, and can result in the generation of toxic byproducts [9]. In using these methods, 26 liters of acidic manganese-containing waste is produced for every 180 grams of product prepared. If the manganese by-products are not carefully removed, the material produced is highly pyrophoric.
Consequently, what is needed is scalable, industrially relevant quantities of oxidized graphite without the generation of acidic or metal-containing waste. However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how the limitations of the art could be overcome.